The invention relates to a method for tightening a screw connection to a predetermined tightening level and an electric screw driving tool.
In particular, a predetermined torque level or a predetermined prestressing force level is preset, which should be achieved after the tightening of the screw connection. This is achieved by using a hand-held screw driving tool with a regulated drive unit and/or control functionality, in particular an electric screwdriver. After a screwing-in phase, a tightening phase begins in which the screw head rests against the bearing surface of the screw connection.
According to the prior art, a separate drive unit regulator can be provided for this purpose, which adjusts the energy supply to the electric screwdriver and regulates its speed. A control unit is also used, which controls the speed, torque, rotation direction, and similar parameters until the screw driving procedure has achieved the predetermined torque level. Since no torque change or only a slight one occurs during the screwing-in phase, in which the screw is merely being screwed into the threaded hole and the screw head is not yet resting against the bearing surface of the screw connection, the speed during the screwing-in phase, for example, is practically constant. With the beginning of the tightening phase, the speed is as a rule reduced with such a method. It then remains virtually constant, practically until the achievement of the predetermined torque level. When the desired torque level is achieved, the control unit interrupts the energy supply from the drive unit regulator to the electric tool. The deactivation precision of the electric tool therefore depends on the reaction time of the system and on the shutoff lag time of the screw after the energy supply to the electric tool is switched off. The lower the tightening speed is, the less torque lag time there generally is; this will be discussed in greater detail later. Low speeds, however, have a negative impact on the duration of the process. Also, changes in the screw joint hardness that occur during the process (e.g. due to different screw batches and coefficients of friction/thread tolerances) are not taken into account in this method.
The disadvantage of the method mentioned at the beginning lies in the fact that due to the predetermined, static, and non-adaptive control unit behavior, the method is usually not flexible, for example with regard to various, alternating screw joint hardnesses; in addition, such a method is not very operator-friendly from an ergonomic standpoint.